.box_4 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  background: aqua url(../oneemoji.jpg) no-repeat top left/5px 5px;
  animation: bg 5s infinite;
}

@keyframes bg {
  50% {
    background: coral bottom right/50px 50px;
  }
}